As used in the well drilling, servicing and producing industry, a blowout preventer is a device which can be attached to a well casing to seal off the annular space between the casing and a polish rod or tubing within the casing so as to contain and control the flow of liquids and/or gases under the pressurized conditions that may be encountered when drilling, operating or servicing the well. Similarly, the device can be attached to a well tubing to seal off the annular space between the tubing and a polish rod or smaller tubing within the first tubing. A conventional type of blowout preventer comprises a housing with a pair of opposing rams mounted in opposite sides of the housing for movement toward or away from each other inside of the housing. The rams can be activated in any suitable manner, either manually or automatically. Sealing blocks are removably connected to the ends of the rams within the housing. The sealing blocks are suitably sized and shaped to engage and seal against the rod or tubing extending coaxially with the well casing or tubing as well as seal against the interior surface of the well casing or tubing. When there is no rod or small tubing within the well casing or tubing, the seals may also be designed to meet fully, without leaving a cylindrical opening therethrough, and thereby seal off the entire top of the well casing or large tubing.
In some environments, the blowout preventer must be able to effect and maintain a seal at elevated temperatures as well as at high pressures. One blowout preventer used commercially for high temperature, high pressure environments employs ram seal elements formed from a composite of asbestos cloth impregnated with low molecular weight suspension grade polytetrafluoroethane. While this blowout preventer performs satisfactorily at moderate conditions such as below 575.degree. F. and 1500 psig, problems can be encountered at more severe conditions. At temperatures in excess of 600.degree. F., which are encountered in some steam injection wells and in some geothermal wells, the polytetrafluoroethylene tends to decompose to fluorocarbon gas, leaving a soggy mass of asbestos fibers and partially decomposed polymer. In addition to possible environmental problems relating to asbestos fibers and fluorocarbon gases, the loss of the seal can result in severe damage to the well and related equipment.
Accordingly, it is an object of the invention to provide a new and improved seal structure. It is an object of the invention to provide a seal which maintains its integrity at elevated temperatures. Another object of the invention is to provide a seal structure which provides an effective seal at lower temperatures as well as at elevated temperatures. A further object of the invention is to provide a seal structure which will reseal after being exposed to elevated temperatures under sealing conditions and then opened and cooled to lower temperature conditions.